Effects of Hydrostatic Pressure on the Compressive Properties of Laminated, 0° Unidirectional, Graphite Fiber/Epoxy Matrix Thick-Composite

Abstract

Compression tests were performed on laminated, 0° continuous, unidirectional, graphite fiber/epoxy matrix thick-composites at various superposed hydrostatic pressures to 4 kbar (400 MPa) at room temperature. Cylindrical samples were made by laminating continuous filament prepreg tapes (3M, SP-319) with fibers oriented along the direction of the cylinder axis. The samples were cured at 135°C for 4 hours in a platen press and post-cured at 135°C for 4 hours in a vacuum oven. The samples contained 60% fiber by volume. The tests were conducted in a high pressure apparatus which was capable of containing pressures to 7 kbar (700 MPa) and of operating at temperatures between −100 and 300°C. The test results show that the compressive stress-strain curves undergo significant changes with increasing pressure. The stress-strain curves show perfectly linear elastic behavior at lower pressure range but exhibit curvature toward the end of the curves at higher pressure range. The compressive modulus obtained from the stress-strain curves increases bilinearly with increasing pressure with a discontinuity located at ∼2 kbar. This continuity has been associated with the low temperature secondary glass transition which is being shifted to room temperature from sub-zero temperature by the applied pressure. The modulus increases from 23.84 GPa at atmospheric pressure to 26.60 GPa at 4 kbar. The increase of the modulus at high pressure is due to finite compressive deformation of matrix by high pressure. Both fracture strength and strain increase linearly with pressure. The fracture strength increases from 376 MPa at atmospheric pressure to 607 MPa at 4 kbar. One of the important findings of our experimental study is that the compressive properties of the thick-composite are far less than the tensile properties of prepreg tapes. It was apparent that fracture of samples occurred by end-crushing due to a combination of fiber buckling, delamination of plies in the transverse direction, and kinking at all pressure levels. High pressure makes it more difficult for the fracture of the samples to be initiated; thereby the fracture strength increases with increasing pressure.